The 68HC812 is intended primarily for expanded-mode operation, with external program memory, and boasts an easy-to-use non-multiplexed memory expansion bus. It has page registers that support up to 4MB Program space, 2MB Data space, and up to 256K Extra space. In a typical design, these memory spaces are interfaced to Flash, RAM, and memory-mapped I/O devices, respectively.

The '912 has on-chip Flash memory, and is primarily intended for single-chip mode opeartion. However, it has a multiplexed address/data bus, and can be used in expanded mode in certain circumstances. Usually, external RAM is used to emulate internal Flash during code development, so that program loading is faster, and the debugger can set unlimited breakpoints in the program in RAM.